Minor planet

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Minor planets (or planetoid), are objects in the solar system that orbit the Sun like planets, but which are smaller than planets and not counted among them. The most common types are asteroids, comets, and trans-Neptunian objects.

Minor planets are divided into groups and families based on their orbital characteristics. It is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are much "tighter" and result from the catastrophic breakup of a large parent asteroid sometime in the past. The only prominent families are Eos asteroids (mean orbital radius = 3.1 AU, eccentricity = 0.1, inclination = 10°) Themis asteroids (mean orbital radius = 3.1 AU, eccentricity = 0.1, inclination = 1°), and Koronis asteroids (mean orbital radius = 2.87 AU, eccentricity = 0.05, inclination = 1°).

Table of contents

1 Groups out to the orbit of Earth
2 Groups out to the orbit of Mars
3 Groups out to the orbit of Jupiter
4 Groups beyond the orbit of Jupiter
5 See also
6 External links

Groups out to the orbit of Earth

There are relatively few asteroids that orbit close to the Sun. Several of these groups are hypothetical at this point in time, with no members having yet been discovered; as such, the names they have been given are provisional.

Vulcanoid asteroids are hypothetical asteroids with an aphelion less than 0.4 AU, ie, they orbit entirely within the orbit of Mercury. A few searches for Vulcanoids have been conducted but there have been none discovered so far.
Apoheless are asteroids whose aphelion is less than 1 AU, meaning they orbit entirely within Earth's orbit. "Apohele" is Hawaiian for "orbit". Other proposed names for this group are Inner-Earth Objects (IEOs) and Anons (as in "Anonymous"). Only one candidate member of this group has been detected so far; 1998 DK₃₆, by David Tholen.
Arjuna asteroids are somewhat vaguely defined as having orbits similar to Earth's; ie, with an average orbital radius of around 1 AU and with low eccentricity and inclination. Due to the vagueness of this definition some asteroids belonging to the Apohele, Amor, Apollo or Aten groups can also be classified as Arjunas.
Earth Trojanss are asteroids located in the Earth-Sun L₄ and L₅ pointss. Their location in the sky as observed from Earth's surface would be fixed at about 60 degrees east and west of the Sun, and as people tend to search for asteroids at much greater elongations few searches have been done in these locations. No Earth trojans are currently known.

Groups out to the orbit of Mars

The Aten, Apollo and Amor asteroids orbit between Earth and Mars.
Mars-crosser asteroids have orbits that cross those of Mars.
The only Mars Trojanss detected have been based on one-time apparitions, which are not as reliable as asteroids with confirmed orbits. The Minor Planets Center has not listed any Mars trojans with confirmed orbits [1].

Groups out to the orbit of Jupiter

A large number of asteroids have orbits between the orbits of Mars and Jupiter, roughly 2 to 4 AU, in a region known as the Main belt. These couldn't form a planet due to the gravitational influence of Jupiter. Jupiter's gravitational influence also results in Kirkwood gaps in the asteroid belt, orbits cleared by orbital resonance. As a result of these gaps the asteroids in this region are divided into a large number of groups. They are:

Hungarias asteroids, with a mean orbital radius between 1.78 AU and 2 AU, an eccentricity less than .18, and inclination between 16° and 34°. These are just outside Mars orbit, and are possibly attracted by the 2:9 resonance.
Phocaeas asteroids, with a mean orbital radius between 2.25 AU and 2.5 AU, an eccentricity greater than .1, and inclination between 18° and 32°. Some sources group the Phocaeas asteroids with the Hungarias, but the division between the two groups is real and caused by the 1:4 resonance with Jupiter.
Floras asteroids have a mean orbital radius between 2.1 AU and 2.3 AU with an inclination of less than 11°.
Nysas asteroids have a mean orbital radius between 2.41 AU and 2.5 AU, an eccentricity between .12 and .21, and an inclination between 1.5° and 4.3°.
Main Belt I asteroids have a mean orbital radius between 2.3 AU and 2.5 AU and an inclination of less than 18°. This group appears to be a catch-all that includes everything in the inner main belt that doesn't belong to the Nysa or Flora groups., with the division at 2.3 AU apparently an arbitrary one without physical significance.
Alinda asteroids have a mean orbital radius of 2.5 AU and an eccentricity between .4 and .65 (approximately). These objects are held by the 1:3 resonance with Jupiter.
Pallas asteroids have a mean orbital radius between 2.5 AU and 2.82 AU and an inclination between 33° and 38°.
Marias asteroids have a mean orbital radius between 2.5 AU and 2.706 AU and an inclination between 12° and 17°.
Main Belt II asteroids have a mean orbital radius between 2.5 AU and 2.706 AU and an inclination less than 33°.
Main Belt IIb asteroids have a mean orbital radius between 2.706 AU and 2.82 AU and an inclination less than 33°.
Koronis asteroids have a mean orbital radius between 2.83 AU and 2.91 AU, an eccentricity less than .11, and an inclination less than 3.5°.
Eos asteroids have a mean orbital radius between 2.99 AU and 3.03 AU, an eccentricity between .01 and .13, and an inclination between 8° and 12°.
Main Belt IIIa asteroids have a mean orbital radius between 2.82 AU and 3.03 AU, an eccentricity less than .35, and an inclination less than 30°.
Themis asteroids have a mean orbital radius between 3.08 AU and 3.24 AU, an eccentricity between .09 and .22, and an inclination less than 3°.
Griqua asteroids have an orbital radius between 3.1 AU and 3.27 AU and an eccentricity greater than .35. These asteroids are in stable 2:1 libration with Jupiter, in high-inclination orbits. There are about 5 to 10 of these known so far, with 1362 Griqua and 8373 Stephengould the most prominent.
Main Belt IIIb asteroids have a mean orbital radius between 3.03 AU and 3.27 AU, an eccentricity less than .35, and an inclination less than 30°.
Cybele asteroids have a mean orbital radius between 3.27 AU and 3.7 AU, an eccentricity less than .3, and an inclination less than 25°. This group appears to cluster around the 4:7 resonance with Jupiter.
Hildas asteroids have a mean orbital radius between 3.7 AU and 4.2 AU, an eccentricity greater than .07, and an inclination less than 20°. These asteroids are in a 2:3 resonance with Jupiter.
Thule asteroids appear to consist of only one object, 279 Thule, in a 3:4 resonance with Jupiter.
Trojan asteroids have a mean orbital radius between 5.05 AU and 5.4 AU, and lie in elongated, curved regions around the two Lagrangian points 60° ahead and behind of Jupiter. The leading point, L₄, is called the 'Greek' node and the trailing L₅ point is called the 'Trojan' node, after the two opposing camps of the legendary Trojan War; with one exception apiece, objects in each node are named for members of that side of the conflict. 617 Patroclus in the Trojan node and 624 Hektor in the Greek node are "misplaced" in the enemy camps.

Between the Hildas and the Trojans (roughly 4.05 AU to 5.0 AU), there's a 'forbidden zone'. Aside from Thule and five objects in unstable-looking orbits, Jupiter's gravity has swept everything out of this region.

Groups beyond the orbit of Jupiter

Most of the minor planets beyond the orbit Jupiter are believed to be composed of ices and other volatiles. Many are similar to comets, differing only in that the perihelia of their orbits are too distant from the Sun to produce a significant tail.

Damocloid asteroids, also known as the "Oort cloud group," are named after 5335 Damocles. They are defined to be objects that have "fallen in" from the Oort cloud, so their aphelia are generally still out past Uranus, but their perihelia are in the inner solar system. They have high eccentricities and sometimes high inclinations, including retrograde orbits. The definition of this group is somewhat fuzzy, and may overlap significantly with comets.
Centaurs have a mean orbital radius roughly between 5.4 AU and 30 AU. They are currently believed to be Trans-Neptunian Objects that "fell in" after encounters with gas giants. The first of these to be discovered was 2060 Chiron.
The Neptune Trojans currently consist of only one object, 2001 QR₃₂₂.
Trans-Neptunian Objects (TNOs) are anything with a mean orbital radius greater than 30 AU. This classification includes the Kuiper Belt Objects (KBOs) and the Oort Cloud.
- Kuiper Belt Objects extend from roughly 30 AU to 50 AU and are broken into the following subcategories:
  - Plutinos are KBOs in a 2:3 resonance with Neptune, just like Pluto. The perihelion of such an object tends to be close to Neptune's orbit (much as happens with Pluto), but when the object comes to perihelion, Neptune alternates between being 90 degrees ahead of and 90 degrees behind of the object, so there's no chance of a collision. The MPC defines any object with a mean orbital radius between 39 AU and 40.5 AU to be a Plutino.
  - Cubewanos, also known as "classical KBOs". They are named after 1992 QB₁ and have a mean orbital radius between approximately 40.5 AU and 47 AU. Cubewanos are objects in the Kuiper belt that didn't get scattered and didn't get locked into a resonance with Neptune.
  - Additional groups exist for other orbital resonances with Neptune than the 2:3 resonance of the Plutinos and the 1:1 resonance of the Neptune Trojans, but they have not yet been officially named. There are several known objects in the 2:1 resonance, unofficially dubbed "Twotinos," with a mean orbital radius of roughly 48 AU and an eccentricity of .37. There are several objects in the 2:5 resonance (mean orbital radius of 55 AU), and objects in the 4:5, 4:7, 3:5, and 3:4 resonances.
  - Scattered Disk Objects (SDOs) generally have very large orbits of up to a few hundred AU. They are assumed to be objects that encountered Neptune and were "scattered" into long-period, very elliptical orbits with perihelia that are still not too far from Neptune's orbit.
- The Oort Cloud is a hypothetical cloud of comets with a mean orbital radius between approximately 50,000 AU and 100,000 AU. No Oort Cloud objects have been detected, the existence of this classification is only inferred from indirect evidence. Some astronomers have tentatively associated Sedna with the Oort cloud.

External links

The Solar System

(For other objects and regions, see: List of solar system objects, Astronomical objects)